tcp: use PRR to reduce cwin in CWR state

Use proportional rate reduction (PRR) algorithm to reduce cwnd in CWR state,
in addition to Recovery state. Retire the current rate-halving in CWR.
When losses are detected via ACKs in CWR state, the sender enters Recovery
state but the cwnd reduction continues and does not restart.

Rename and refactor cwnd reduction functions since both CWR and Recovery
use the same algorithm:
tcp_init_cwnd_reduction() is new and initiates reduction state variables.
tcp_cwnd_reduction() is previously tcp_update_cwnd_in_recovery().
tcp_ends_cwnd_reduction() is previously  tcp_complete_cwr().

The rate halving functions and logic such as tcp_cwnd_down(), tcp_min_cwnd(),
and the cwnd moderation inside tcp_enter_cwr() are removed. The unused
parameter, flag, in tcp_cwnd_reduction() is also removed.

Signed-off-by: Yuchung Cheng <ycheng@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 3 insertions, 3 deletions

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 9383b51f3efc..cfe6ffe1c177 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -2037,10 +2037,10 @@ static bool tcp_write_xmit(struct sock *sk, unsigned int mss_now, int nonagle,
                 if (push_one)
                         break;
         }
-        if (inet_csk(sk)->icsk_ca_state == TCP_CA_Recovery)
-                tp->prr_out += sent_pkts;
 
         if (likely(sent_pkts)) {
+                if (tcp_in_cwnd_reduction(sk))
+                        tp->prr_out += sent_pkts;
                 tcp_cwnd_validate(sk);
                 return false;
         }
@@ -2542,7 +2542,7 @@ begin_fwd:
                 }
                 NET_INC_STATS_BH(sock_net(sk), mib_idx);
 
-                if (inet_csk(sk)->icsk_ca_state == TCP_CA_Recovery)
+                if (tcp_in_cwnd_reduction(sk))
                         tp->prr_out += tcp_skb_pcount(skb);
 
                 if (skb == tcp_write_queue_head(sk))